VSE-Background image behind foreground video. Alpha over, alpha under don't work
Your PNGs were probably not exported with the alpha channel.

Make sure you export the PNGs as RGBA



To check if  your image has an alpha channel load it in the UV/Image editor.

An image with alpha channel will look like this, with a checker pattern on the alpha:



An image with no alpha channel will not show that pattern, but a solid color:



To make the alpha channel transparent on the VSE, select the strip with the pngs, presss N to make the panel visible, and in the blend type select alpha over.